Radio-frequency (RF) antennas may be used in various applications where it is desired to radiate electromagnetic (EM) energy into a reservoir filled with a target material in order to change one or more of the target material's characteristics. For example, radiation of EM energy may initiate or enhance a chemical process or reaction, heat the target material, or help to perform an analysis of the target material's physical properties or composition. EM radiation may be used at different stages of oil production: heating the soil to decrease viscosity of oil, improve oil's mobility, or upgrade the bitumen or heavy oil in a process on the surface or underground.
The RF antenna and its environment form a connected system, and the RF antenna's performance greatly depends on the EM properties of the surrounding target material. The input impedance of the RF antenna characterizes its ability to deliver high RF power to the target material. The RF antenna is a part of an electrical circuit that typically comprises an RF generator, impedance matching circuits, and a feed transmission line. If there is a mismatch between the RF antenna's input impedance and its feed system, at least one part of the EM power will be reflected from the antenna back to the RF generator. This EM power reflection reduces the amount of power delivered by the RF antenna to the target material in the reservoir and increases losses and/or heating in the transmission line and the RF generator. This typically leads to a decrease in the overall system efficiency.
To reduce or eliminate the EM power reflection, impedance matching circuits are generally used between the RF antenna and the RF generator. The matching circuits tend to be expensive and complex and typically operate optimally only within a narrow frequency range. Instead, dynamic or adaptive impedance matching circuits that match the antenna to the feed transmission line over a wider range of frequencies and/or values of input impedance, may be used. However, the cost and complexity of such dynamic matching circuits is significantly higher than that of the regular matching circuits.
It would be desirable to reduce the EM power reflection from the RF antenna delivering the RF power into surrounding target material by improving the impedance matching between the RF antenna and the feed transmission line without the use of an impedance matching circuit.